The present invention relates to aqueous starch dispersions and more particularly to starch dispersions containing glyoxal compounds and having coatable viscosity, good storage stability and which provides a film having good tensile strength and elongation properties.
Industrial starch may be utilized in a wide variety of applications including as coatings for paper or paper board. It desirably has good tensile and elongation properties, both under wet and dry conditions. Starch compositions may desirably be prepared in the form of coatable aqueous dispersions capable of being drawn down and cured on suitable substrates. The end user of these compositions may utilize them in a wide variety of applications by simply removing solvent to produce strong water resistant films.
For many commercial applications starch is gelatinized by the end user prior to being used. Gelatinization occurs after starch granules are dispersed as a slurry in water with the resultant aqueous slurry being heated to over 50.degree. C. and usually over about 65.degree. C. Under such conditions starch grains tend to absorb water, swell, and eventually rupture to allow starch fragments and molecules to disperse in water. This rupturing and dispersion is generally referred to as "gelatinization" and is an irreversible reaction resulting in a relatively thick starch dispersion. The ability to gelatinize starch on site just prior to its end use is undesirable for many end users because the additional equipment needed to handle and cook the starch is impractical or uneconomical.
A pre-gelatinized starch in liquid form would be desirable for certain end users as it would eliminate the need to handle dry starch and eliminate a cooking and dispersing step in the process. A problem with such pre-gelatinized starch is its stability over time. Because the product may spend many days in distribution and transportation, and because it sometimes is stored for a time prior to use at its ultimate destination, it is important that the product have a somewhat extended storage life wherein it maintains its desirable qualities and characteristics, particularly coatable viscosity. Prior art gelatinized starch products containing amylose tend to undergo a process known as retrogradation which results in the formation of a gel or precipitate. The viscosity also tends to thicken over time to a point which renders the products unusable for many commercial purposes. The problem is that by the time the pre-gelatinized starch product has been transported to its end user and prepared for use, it has often thickened to a point where it is no longer of a coatable viscosity. A preferred viscosity is about 350 centipoise or less. Viscosities above about 1000 centipoise are not readily flowable and are generally not usable in products such as coatings.
The cross-linking of starches with multifunctional reagents which are reactive with starch hydroxyl groups is well known. Glyoxals and polyaldehyde compounds and resins have been previously utilized as cross-linkers. The simple mixing of glyoxal with a starch dispersion will provide a gel. U.S. Pat. No. 4,455,416 describes a paper coating containing starch binder and a cyclic urea/glyoxal/polyol condensate as an insolubilizer for the binder. As an insolubilizer the glyoxal condensate is inactive until the coating is applied and cured upon drying whereupon the glyoxal crosslinks the starch to impart water resistance. U.S. Pat. No. 4,021,260 describes ethoxylated fatty alcohols as starch viscosity control agents. U.S. Pat. No. 3,324,057 discloses the use of dialdehyde starch in the preparation of paper coatings. U.S. Pat. No. 3,740,391 describes the production of aqueous dispersions of a starch first reacted with an acrylamide and, subsequently, with glyoxal. U.S. Pat. No. 4,013,629 discloses a glyoxal binder system. U.S. Pat. No. 4,425,452 discloses coating paper material via an enzymatically converted starch. British Patent No. 2017124 discloses polysaccharides cross-linked with glyoxal.